Not just cash. I suppose we could go pretty in depth when it comes to Ishihara's motivations, his overwhelming lust for violence, and his craving to humiliate (as stated above, when the six first arrived, it wasn't exactly necessary to jam rods up their asses).
By what we've seen so far the doctor and Ishihara seem to have that place locked down pretty tight, and back in those days prisons, mental institutions and similar facilities ran largely unregulated, evident by their deplorable conditions. It looks like whoever is really in charge of the place is either in collusion with the bad guys or rarely visits the prison.